1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor (HBT) and a method for fabricating an HBT, and more particularly to an HBT with self-aligned base metal contacts utilizing a double photoresist, which requires fewer process steps than known methods, while minimizing damage to the active emitter contact region.
2. Description of the Prior Art
Processes for fabricating heterojunction bipolar transistors (HBTs) are known in the art. Examples of such processes are disclosed in U.S. Pat. Nos. 5,298,439 and 5,344,786, hereby incorporated by reference. An important consideration in fabricating HBTs is the method for forming the base metal contacts to emitter mesa spacing in a minimal, reproducible method.
The '786 patent discloses the use of a dielectric material formed by chemical vapor deposition on top of an emitter layer. A photoresist is spun on top of the dielectric layer. An emitter dielectric island is formed by reactive ion etching. Another dielectric is deposited over the emitter dielectric island forming protective side walls and a dielectric layer on top of the flat surface of the semiconductor. The side walls, formed on the emitter dielectric island, overhang the emitter mesa and thus serve to locate the best base metal contacts on the base layer away from the emitter mesa. The dielectric layer on the flat surface of the semiconductors is removed by anisotropic etching. In order to locate the base metal contacts, a photoresist is used. Base metal is evaporated on top. The dielectric also acts as a mask for the base metal contacts. Removal of the photoresist lifts off the base metal except for the masked portion to form the base metal contacts.
The '439 patent discloses the use of an emitter metal, patterned on a portion of the emitter contact layer by photolithography. The emitter metal is used as a mask for etching the emitter contact layer in a portion of the emitter layer beneath, to form an emitter mesa. A photoresist is spun over the emitter mesa to enable the remaining portion of the emitter to be exposed. Etching is used to expose the base layer to enable the base metal contacts to be formed.
In both the '439 and '786 patents, a dielectric or ohmic metal is used as a mask for the emitter mesa. Unfortunately, both of these techniques require a separate process step to etch the dielectric or metal mask after the mesa is formed. In addition, etching of the insulator or base metal exposes an active emitter contact region to an etching process, which can cause damage.